The present invention relates to an image reading apparatus and, more particularly, to an image reading apparatus which is applied to a film scanner system for reading color images and has a shading correction function for performing shading correction and the like.
Conventional image reading apparatuses such as a film scanner system for reading color images use a color linear sensor such as a CCD to read color images.
Unfortunately, in image reading apparatuses using a color linear sensor such as a CCD, the sensitivity varies from one pixel to another due to, e.g., an uneven density of RGB filters, an uneven photoelectric conversion efficiency, or an uneven transfer efficiency. This significantly deteriorates the quality of read images.
Such unevenness is generally called shading.
This shading is produced not only by a color linear sensor such as a CCD described above but also by the nonuniformity of luminance of an illuminating light source itself. This nonuniformity results from the shadow of mercury grains of a fluorescent lamp used as a light source for illuminating color images or from uneven coating of a phosphor.
One conventional method (to be referred to as a shading correction hereinafter) of eliminating the influence of this shading is to prestore shading patterns and correct shading by multiplying a sensor output by an inverse coefficient on the basis of the stored shading patterns.
In the shading correction as described above, however, if color originals have substantially different optical characteristics, no equally optimal corrections can be performed for these color originals.
For example, when color originals are a negative film and a positive film, the quality of an image deteriorates upon shading correction of the positive film if shading correction of the negative film is optimized.
Jpn. Pat. Appln. KOKAI Publication No. 61-121667, therefore, has disclosed a method of performing different shading correction processes for a negative film and a positive film by checking whether the color original is a negative film and a positive film.
Unfortunately, the shading correction processes as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-121667 require a memory with a very large capacity to store shading parameters.
Accordingly, the method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-121667 wastes a very large memory capacity.